Autor:innen:
Susanne K. Woche | Institut für Bodenkunde, Leibniz Universität Hannover | Germany
Marc-O. Göbel | Institut für Bodenkunde, leibniz Universität Hannover | Germany
Jörg Bachmann | Institut für Bodenkunde, Leibniz Universität Hannover | Germany
Soil hydraulic processes and soil wetting properties are strongly interrelated. Water repellency can cause formation of preferential flow pathways and uneven water distribution that, e.g., as well may influence abundance and distribution of microorganisms (MO). Modification of soil wettability, quantified in terms of contact angle (CA), thus is suggested to impact the soil water regime and related soil parameters. A preceding study [1] revealed heat treatment at 80°C to distinctly increase CA from < 100° to > 120°. Here, we went beyond and tested the impact of heat treatment from 80°C to 600°C on two sandy topsoils with original CA < 100° (Ut) and > 100° (Ct). Both materials reached maximum CA at 105°C-treatment (> 130°) that did not change until 180°C-treatment. At 230°C-treament, CA decreased distinctly to around 60°. At 360°C-treatment, Ct was wettable (CA=0°) while Ut showed CA of 0° - 25° up to 600°C-treatment, indicating irregularly distributed heat-resistant components. In agreement with observed CA, the base component of surface free energy (SFE) decreased at 105°C-treatment and increased between 230°C- and 280°C-treatment. Further, loss on ignition (LOI) increased by about 100% and the up to 180°C-treatment constant C/N ratio decreased by about 43% at 230°C-treatment. An increase in pH by about one unit indicated ash formation to start at 280°C-treatment. Summarized, heat treatment from 80°C to 180°C resulted in persistently hydrophobic interfaces, while the interval from 230°C to 280°C marked a turning point that fundamentally changed physical and chemical interface properties by volatilization of particle coatings as shown by XPS spectra [2]. The findings are potentially relevant for natural soil systems in context with global climate change with expected higher frequencies of not only droughts, but as well wildfires that can heat up the soil surface layer to the temperatures tested here [3]. In conclusion, within the affected soil layer, exposure to 80°C - 180°C may induce or enhance hydrophobicity, while at exposure to temperatures ≥ 230°C particle interfaces will turn hydrophilic, with probably as well negative consequences for MO abundance as volatilization of the organic particle coating results in bare, basically nutrient-free mineral surfaces.
[1] Bachmann et al., 2021, doi: 10.1111/ejss.13102
[2] Gaj et al., 2019, doi: 10.2136/vzj2018.04.0083
[3] Martinez et al., 2022, doi: 10.1016/j.scitotenv.2022.153654